The present invention relates to hydrocarbon-acid emulsion compositions and methods of using such emulsion compositions in subterranean applications.
Acidizing and fracturing treatments using aqueous acidic solutions commonly are carried out in subterranean formations (including those that contain hydrocarbons as well as those that do not) to accomplish a number of purposes, one of which may be to increase the productivity of the formation.
Acidizing techniques can be carried out as “matrix acidizing” procedures or as “acid fracturing” procedures. Generally, in acidizing treatments, aqueous acidic solutions are introduced into the subterranean formation under pressure so that the acidic solution flows into the pore spaces of the formation to remove near-well formation damage and other damaging substances. The term “damage” as used herein refers to undesirable deposits that may reduce permeability, including, but not limited to, scale, skin, hydrates, geological deposits, deposits from stimulation operations, and/or condensate banking. The acidic solution reacts with acid-soluble materials contained in the formation which may result in an increase in the size of the pore spaces and an increase in the permeability of the formation. This procedure commonly enhances production by increasing the effective well radius. When performed at pressures above the pressure required to fracture the formation, the procedure is often referred to as acid fracturing. Fracture-acidizing involves the formation of one or more fractures in the formation and the introduction of an aqueous acidizing fluid into the fractures to etch the fractures' faces whereby flow channels are formed when the fractures close. The aqueous acidizing fluid also enlarges the pore spaces in the fracture faces and in the formation. The use of the term “acidizing” herein refers to both types of acidizing treatments, and more specifically, refers to the general process of introducing an acid down hole to perform a desired function, e.g., to acidize a portion of a subterranean formation or any damage contained therein.
A variety of acid compositions, including acid-in-oil emulsions, may be used in these acidizing treatments. The use of the term “emulsion” herein refers to a dispersion or suspension of one immiscible liquid in another creating an external phase and an internal phase. In some instances, “acid-in-oil emulsions,” wherein the internal phase is an acid and the external phase is a hydrocarbon, have heretofore been used to decrease the rates of acid diffusion and to increase the distance that unspent acid can travel within a formation.
The external oil phase of such acid-in-oil emulsions is typically crude oil or diesel fuel, which provides a barrier to the diffusion and spending of the acid, theoretically allowing unspent acid to be delivered deeply and uniformly into a formation. While the use of crude oil and/or diesel fuel as the external phase in an emulsion can be useful in some instances, it may also have significant drawbacks and disadvantages. For example, the use of diesel fuel may introduce numerous additives that can have an adverse affect on the wetability of the formation, which may result in reduced permeability. Similarly, the use of crude oil introduces impurities, such as paraffin and asphaltenes, into the formation that can reduce the permeability of the formation and ultimately decrease well production.
An additional problem of great concern that may contribute to a decrease in the productivity of a well is condensate banking. The term “condensate banking” and/or “condensate blockage” as used herein refers to the precipitation and/or accumulation of a liquid condensate in a region near a well bore or adjacent to a fracture face, which may occur as a result of, inter alia, gas production from a reservoir flowing at a bottom hole pressure lower than the dewpoint pressure. When stimulation techniques, such as those mentioned above utilizing acid-in-oil emulsions, are performed in dry gas reservoirs and/or subterranean formations that contain condensate blockages, the oil external phase of the emulsion may further reduce the permeability of the formation due in part to the presence of low molecular weight components in the oil external phase, which may not evaporate and thereby lead to phase trapping.